| page.title=Audio Terminology |
| @jd:body |
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| <div id="qv-wrapper"> |
| <div id="qv"> |
| <h2>In this document</h2> |
| <ol id="auto-toc"> |
| </ol> |
| </div> |
| </div> |
| |
| <p> |
| This document provides a glossary of audio-related terminology, including |
| a list of widely used, generic terms and a list of terms that are specific |
| to Android. |
| </p> |
| |
| <h2 id="genericTerm">Generic Terms</h2> |
| |
| <p> |
| These are audio terms that are widely used, with their conventional meanings. |
| </p> |
| |
| <h3 id="digitalAudioTerms">Digital Audio</h3> |
| |
| <dl> |
| |
| <dt>acoustics</dt> |
| <dd> |
| The study of the mechanical properties of sound, for example how the |
| physical placement of transducers such as speakers and microphones on |
| a device affects perceived audio quality. |
| </dd> |
| |
| <dt>attenuation</dt> |
| <dd> |
| A multiplicative factor less than or equal to 1.0, |
| applied to an audio signal to decrease the signal level. |
| Compare to "gain". |
| </dd> |
| |
| <dt>bits per sample or bit depth</dt> |
| <dd> |
| Number of bits of information per sample. |
| </dd> |
| |
| <dt>channel</dt> |
| <dd> |
| A single stream of audio information, usually corresponding to one |
| location of recording or playback. |
| </dd> |
| |
| <dt>downmixing</dt> |
| <dd> |
| To decrease the number of channels, e.g. from stereo to mono, or from 5.1 to stereo. |
| This can be accomplished by dropping some channels, mixing channels, or more advanced signal processing. |
| Simple mixing without attenuation or limiting has the potential for overflow and clipping. |
| Compare to "upmixing". |
| </dd> |
| |
| <dt>duck</dt> |
| <dd> |
| To temporarily reduce the volume of one stream, when another stream |
| becomes active. For example, if music is playing and a notification arrives, |
| then the music stream could be ducked while the notification plays. |
| Compare to "mute". |
| </dd> |
| |
| <dt>frame</dt> |
| <dd> |
| A set of samples, one per channel, at a point in time. |
| </dd> |
| |
| <dt>frames per buffer</dt> |
| <dd> |
| The number of frames handed from one module to the next at once; |
| for example the audio HAL interface uses this concept. |
| </dd> |
| |
| <dt>gain</dt> |
| <dd> |
| A multiplicative factor greater than or equal to 1.0, |
| applied to an audio signal to increase the signal level. |
| Compare to "attenuation". |
| </dd> |
| |
| <dt>Hz</dt> |
| <dd> |
| The units for sample rate or frame rate. |
| </dd> |
| |
| <dt>latency</dt> |
| <dd> |
| Time delay as a signal passes through a system. |
| </dd> |
| |
| <dt>mono</dt> |
| <dd> |
| One channel. |
| </dd> |
| |
| <dt>multichannel</dt> |
| <dd> |
| See "surround sound". |
| Strictly, since stereo is more than one channel, it is also "multi" channel. |
| But that usage would be confusing. |
| </dd> |
| |
| <dt>mute</dt> |
| <dd> |
| To (temporarily) force volume to be zero, independently from the usual volume controls. |
| </dd> |
| |
| <dt>PCM</dt> |
| <dd> |
| Pulse Code Modulation, the most common low-level encoding of digital audio. |
| The audio signal is sampled at a regular interval, called the sample rate, |
| and then quantized to discrete values within a particular range depending on the bit depth. |
| For example, for 16-bit PCM, the sample values are integers between -32768 and +32767. |
| </dd> |
| |
| <dt>ramp</dt> |
| <dd> |
| To gradually increase or decrease the level of a particular audio parameter, |
| for example volume or the strength of an effect. |
| A volume ramp is commonly applied when pausing and resuming music, to avoid a hard audible transition. |
| </dd> |
| |
| <dt>sample</dt> |
| <dd> |
| A number representing the audio value for a single channel at a point in time. |
| </dd> |
| |
| <dt>sample rate or frame rate</dt> |
| <dd> |
| Number of frames per second; |
| note that "frame rate" is thus more accurate, |
| but "sample rate" is conventionally used to mean "frame rate." |
| </dd> |
| |
| <dt>sonification</dt> |
| <dd> |
| The use of sound to express feedback or information, |
| for example touch sounds and keyboard sounds. |
| </dd> |
| |
| <dt>stereo</dt> |
| <dd> |
| Two channels. |
| </dd> |
| |
| <dt>stereo widening</dt> |
| <dd> |
| An effect applied to a stereo signal, to make another stereo signal which sounds fuller and richer. |
| The effect can also be applied to a mono signal, in which case it is a type of upmixing. |
| </dd> |
| |
| <dt>surround sound</dt> |
| <dd> |
| Various techniques for increasing the ability of a listener to perceive |
| sound position beyond stereo left and right. |
| </dd> |
| |
| <dt>upmixing</dt> |
| <dd> |
| To increase the number of channels, e.g. from mono to stereo, or from stereo to surround sound. |
| This can be accomplished by duplication, panning, or more advanced signal processing. |
| Compare to "downmixing". |
| </dd> |
| |
| <dt>virtualizer</dt> |
| <dd> |
| An effect that attempts to spatialize audio channels, such as trying to |
| simulate more speakers, or give the illusion that various sound sources have position. |
| </dd> |
| |
| <dt>volume</dt> |
| <dd> |
| Loudness, the subjective strength of an audio signal. |
| </dd> |
| |
| </dl> |
| |
| <h3 id="hardwareTerms">Hardware and Accessories</h3> |
| |
| <p> |
| These terms are related to audio hardware and accessories. |
| </p> |
| |
| <h4 id="interDeviceTerms">Inter-device interconnect</h4> |
| |
| <p> |
| These technologies connect audio and video components between devices, |
| and are readily visible at the external connectors. The HAL implementor |
| may need to be aware of these, as well as the end user. |
| </p> |
| |
| <dl> |
| |
| <dt>Bluetooth</dt> |
| <dd> |
| A short range wireless technology. |
| The major audio-related |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Bluetooth_profile" |
| target="_android">Bluetooth profiles</a> |
| and |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Bluetooth_protocols" |
| target="_android">Bluetooth protocols</a> |
| are described at these Wikipedia articles: |
| |
| <ul> |
| |
| <li><a class="external-link" |
| href="http://en.wikipedia.org/wiki/Bluetooth_profile#Advanced_Audio_Distribution_Profile_.28A2DP.29" |
| target="_android">A2DP</a> |
| for music |
| </li> |
| |
| <li><a class="external-link" |
| href="http://en.wikipedia.org/wiki/Bluetooth_protocols#Synchronous_connection-oriented_.28SCO.29_link" |
| target="_android">SCO</a> |
| for telephony |
| </li> |
| |
| </ul> |
| |
| </dd> |
| |
| <dt>DisplayPort</dt> |
| <dd> |
| Digital display interface by VESA. |
| </dd> |
| |
| <dt>HDMI</dt> |
| <dd> |
| High-Definition Multimedia Interface, an interface for transferring |
| audio and video data. For mobile devices, either a micro-HDMI (type D) or MHL connector is used. |
| </dd> |
| |
| <dt>MHL</dt> |
| <dd> |
| Mobile High-Definition Link is a mobile audio/video interface, often |
| over micro-USB connector. |
| </dd> |
| |
| <dt>phone connector</dt> |
| <dd> |
| A mini or sub-mini phone connector |
| connects a device to wired headphones, headset, or line-level amplifier. |
| </dd> |
| |
| <dt>SlimPort</dt> |
| <dd> |
| An adapter from micro-USB to HDMI. |
| </dd> |
| |
| <dt>S/PDIF</dt> |
| <dd> |
| Sony/Philips Digital Interface Format is an interconnect for uncompressed PCM. |
| See Wikipedia article <a class="external-link" href="http://en.wikipedia.org/wiki/S/PDIF" |
| target="_android">S/PDIF</a>. |
| </dd> |
| |
| <dt>USB</dt> |
| <dd> |
| Universal Serial Bus. |
| See Wikipedia article <a class="external-link" href="http://en.wikipedia.org/wiki/USB" target="_android">USB</a>. |
| </dd> |
| |
| </dl> |
| |
| <h4 id="intraDeviceTerms">Intra-device interconnect</h4> |
| |
| <p> |
| These technologies connect internal audio components within a given |
| device, and are not visible without disassembling the device. The HAL |
| implementor may need to be aware of these, but not the end user. |
| </p> |
| |
| See these Wikipedia articles: |
| <ul> |
| <li><a class="external-link" href="http://en.wikipedia.org/wiki/General-purpose_input/output" |
| target="_android">GPIO</a></li> |
| <li><a class="external-link" href="http://en.wikipedia.org/wiki/I%C2%B2C" target="_android">I²C</a></li> |
| <li><a class="external-link" href="http://en.wikipedia.org/wiki/I%C2%B2S" target="_android">I²S</a></li> |
| <li><a class="external-link" href="http://en.wikipedia.org/wiki/McASP" target="_android">McASP</a></li> |
| <li><a class="external-link" href="http://en.wikipedia.org/wiki/SLIMbus" target="_android">SLIMbus</a></li> |
| <li><a class="external-link" href="http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus" |
| target="_android">SPI</a></li> |
| </ul> |
| |
| <h3 id="signalTerms">Audio Signal Path</h3> |
| |
| <p> |
| These terms are related to the signal path that audio data follows from |
| an application to the transducer, or vice-versa. |
| </p> |
| |
| <dl> |
| |
| <dt>ADC</dt> |
| <dd> |
| Analog to digital converter, a module that converts an analog signal |
| (continuous in both time and amplitude) to a digital signal (discrete in |
| both time and amplitude). Conceptually, an ADC consists of a periodic |
| sample-and-hold followed by a quantizer, although it does not have to |
| be implemented that way. An ADC is usually preceded by a low-pass filter |
| to remove any high frequency components that are not representable using |
| the desired sample rate. See Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Analog-to-digital_converter" |
| target="_android">Analog-to-digital_converter</a>. |
| </dd> |
| |
| <dt>AP</dt> |
| <dd> |
| Application processor, the main general-purpose computer on a mobile device. |
| </dd> |
| |
| <dt>codec</dt> |
| <dd> |
| Coder-decoder, a module that encodes and/or decodes an audio signal |
| from one representation to another. Typically this is analog to PCM, or PCM to analog. |
| Strictly, the term "codec" is reserved for modules that both encode and decode, |
| however it can also more loosely refer to only one of these. |
| See Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Audio_codec" target="_android">Audio codec</a>. |
| </dd> |
| |
| <dt>DAC</dt> |
| <dd> |
| Digital to analog converter, a module that converts a digital signal |
| (discrete in both time and amplitude) to an analog signal |
| (continuous in both time and amplitude). A DAC is usually followed by |
| a low-pass filter to remove any high frequency components introduced |
| by digital quantization. |
| See Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Digital-to-analog_converter" |
| target="_android">Digital-to-analog converter</a>. |
| </dd> |
| |
| <dt>DSP</dt> |
| <dd> |
| Digital Signal Processor, an optional component which is typically located |
| after the application processor (for output), or before the application processor (for input). |
| The primary purpose of a DSP is to off-load the application processor, |
| and provide signal processing features at a lower power cost. |
| </dd> |
| |
| <dt>PDM</dt> |
| <dd> |
| Pulse-density modulation |
| is a form of modulation used to represent an analog signal by a digital signal, |
| where the relative density of 1s versus 0s indicates the signal level. |
| It is commonly used by digital to analog converters. |
| See Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Pulse-density_modulation" |
| target="_android">Pulse-density modulation</a>. |
| </dd> |
| |
| <dt>PWM</dt> |
| <dd> |
| Pulse-width modulation |
| is a form of modulation used to represent an analog signal by a digital signal, |
| where the relative width of a digital pulse indicates the signal level. |
| It is commonly used by analog to digital converters. |
| See Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Pulse-width_modulation" |
| target="_android">Pulse-width modulation</a>. |
| </dd> |
| |
| </dl> |
| |
| <h2 id="androidSpecificTerms">Android-Specific Terms</h2> |
| |
| <p> |
| These are terms specific to the Android audio framework, or that |
| may have a special meaning within Android beyond their general meaning. |
| </p> |
| |
| <dl> |
| |
| <dt>ALSA</dt> |
| <dd> |
| Advanced Linux Sound Architecture. As the name suggests, it is an audio |
| framework primarily for Linux, but it has influenced other systems. |
| See Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Advanced_Linux_Sound_Architecture" target="_android">ALSA</a> |
| for the general definition. As used within Android, it refers primarily |
| to the kernel audio framework and drivers, not to the user-mode API. See |
| tinyalsa. |
| </dd> |
| |
| <dt>AudioEffect</dt> |
| <dd> |
| An API and implementation framework for output (post-processing) effects |
| and input (pre-processing) effects. The API is defined at |
| <a href="http://developer.android.com/reference/android/media/audiofx/AudioEffect.html" target="_android">android.media.audiofx.AudioEffect</a>. |
| </dd> |
| |
| <dt>AudioFlinger</dt> |
| <dd> |
| The sound server implementation for Android. AudioFlinger |
| runs within the mediaserver process. See Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Sound_server" target="_android">Sound server</a> |
| for the generic definition. |
| </dd> |
| |
| <dt>audio focus</dt> |
| <dd> |
| A set of APIs for managing audio interactions across multiple independent apps. |
| See <a href="http://developer.android.com/training/managing-audio/audio-focus.html">Managing Audio |
| Focus</a> and the focus-related methods and constants of |
| <a href="http://developer.android.com/reference/android/media/AudioManager.html">android.media.AudioManager</a>. |
| </dd> |
| |
| <dt>AudioMixer</dt> |
| <dd> |
| The module within AudioFlinger responsible for |
| combining multiple tracks and applying attenuation |
| (volume) and certain effects. The Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Audio_mixing_(recorded_music)" target="_android">Audio mixing (recorded music)</a> |
| may be useful for understanding the generic |
| concept. But that article describes a mixer more as a hardware device |
| or a software application, rather than a software module within a system. |
| </dd> |
| |
| <dt>audio policy</dt> |
| <dd> |
| Service responsible for all actions that require a policy decision |
| to be made first, such as opening a new I/O stream, re-routing after a |
| change and stream volume management. |
| </dd> |
| |
| <dt>AudioRecord</dt> |
| <dd> |
| The primary low-level client API for receiving data from an audio |
| input device such as microphone. The data is usually in pulse-code modulation |
| (PCM) format. |
| The API is defined at |
| <a href="http://developer.android.com/reference/android/media/AudioRecord.html" |
| target="_android">android.media.AudioRecord</a>. |
| </dd> |
| |
| <dt>AudioResampler</dt> |
| <dd> |
| The module within AudioFlinger responsible for |
| <a href="audio_src.html">sample rate conversion</a>. |
| </dd> |
| |
| <dt>AudioTrack</dt> |
| <dd> |
| The primary low-level client API for sending data to an audio output |
| device such as a speaker. The data is usually in PCM format. |
| The API is defined at |
| <a href="http://developer.android.com/reference/android/media/AudioTrack.html" |
| target="_android">android.media.AudioTrack</a>. |
| </dd> |
| |
| <dt>client</dt> |
| <dd> |
| Usually same as application or app, but sometimes the "client" of |
| AudioFlinger is actually a thread running within the mediaserver system |
| process. An example of that is when playing media that is decoded by a |
| MediaPlayer object. |
| </dd> |
| |
| <dt>HAL</dt> |
| <dd> |
| Hardware Abstraction Layer. HAL is a generic term in Android. With |
| respect to audio, it is a layer between AudioFlinger and the kernel |
| device driver with a C API, which replaces the earlier C++ libaudio. |
| </dd> |
| |
| <dt>FastMixer</dt> |
| <dd> |
| A thread within AudioFlinger that services lower latency "fast tracks" |
| and drives the primary output device. |
| </dd> |
| |
| <dt>fast track</dt> |
| <dd> |
| An AudioTrack client with lower latency but fewer features, on some devices. |
| </dd> |
| |
| <dt>MediaPlayer</dt> |
| <dd> |
| A higher-level client API than AudioTrack, for playing either encoded |
| content, or content which includes multimedia audio and video tracks. |
| </dd> |
| |
| <dt>media.log</dt> |
| <dd> |
| An AudioFlinger debugging feature, available in custom builds only, |
| for logging audio events to a circular buffer where they can then be |
| dumped retroactively when needed. |
| </dd> |
| |
| <dt>mediaserver</dt> |
| <dd> |
| An Android system process that contains a number of media-related |
| services, including AudioFlinger. |
| </dd> |
| |
| <dt>NBAIO</dt> |
| <dd> |
| An abstraction for "non-blocking" audio input/output ports used within |
| AudioFlinger. The name can be misleading, as some implementations of |
| the NBAIO API actually do support blocking. The key implementations of |
| NBAIO are for pipes of various kinds. |
| </dd> |
| |
| <dt>normal mixer</dt> |
| <dd> |
| A thread within AudioFlinger that services most full-featured |
| AudioTrack clients, and either directly drives an output device or feeds |
| its sub-mix into FastMixer via a pipe. |
| </dd> |
| |
| <dt>OpenSL ES</dt> |
| <dd> |
| An audio API standard by The Khronos Group. Android versions since |
| API level 9 support a native audio API which is based on a subset of |
| OpenSL ES 1.0.1. |
| </dd> |
| |
| <dt>silent mode</dt> |
| <dd> |
| A user-settable feature to mute the phone ringer and notifications, |
| without affecting media playback (music, videos, games) or alarms. |
| </dd> |
| |
| <dt>SoundPool</dt> |
| <dd> |
| A higher-level client API than AudioTrack, used for playing sampled |
| audio clips. It is useful for triggering UI feedback, game sounds, etc. |
| The API is defined at |
| <a href="http://developer.android.com/reference/android/media/SoundPool.html" |
| target="_android">android.media.SoundPool</a>. |
| </dd> |
| |
| <dt>Stagefright</dt> |
| <dd> |
| See <a href="{@docRoot}devices/media.html">Media</a>. |
| </dd> |
| |
| <dt>StateQueue</dt> |
| <dd> |
| A module within AudioFlinger responsible for synchronizing state |
| among threads. Whereas NBAIO is used to pass data, StateQueue is used |
| to pass control information. |
| </dd> |
| |
| <dt>strategy</dt> |
| <dd> |
| A grouping of stream types with similar behavior, used by the audio policy service. |
| </dd> |
| |
| <dt>stream type</dt> |
| <dd> |
| An enumeration that expresses a use case for audio output. |
| The audio policy implementation uses the stream type, along with other parameters, |
| to determine volume and routing decisions. |
| Specific stream types are listed at |
| <a href="http://developer.android.com/reference/android/media/AudioManager.html">android.media.AudioManager</a>. |
| </dd> |
| |
| <dt>tee sink</dt> |
| <dd> |
| See the separate article on tee sink in |
| <a href="audio_debugging.html#teeSink">Audio Debugging</a>. |
| </dd> |
| |
| <dt>tinyalsa</dt> |
| <dd> |
| A small user-mode API above ALSA kernel with BSD license, recommended |
| for use in HAL implementations. |
| </dd> |
| |
| <dt>ToneGenerator</dt> |
| <dd> |
| A higher-level client API than AudioTrack, used for playing DTMF signals. |
| See the Wikipedia article |
| <a class="external-link" href="http://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signaling" |
| target="_android">Dual-tone multi-frequency signaling</a>, |
| and the API definition at |
| <a href="http://developer.android.com/reference/android/media/ToneGenerator.html" |
| target="_android">android.media.ToneGenerator</a>. |
| </dd> |
| |
| <dt>track</dt> |
| <dd> |
| An audio stream, controlled by the AudioTrack API. |
| </dd> |
| |
| <dt>volume attenuation curve</dt> |
| <dd> |
| A device-specific mapping from a generic volume index to a particular attenuation factor |
| for a given output. |
| </dd> |
| |
| <dt>volume index</dt> |
| <dd> |
| A unitless integer that expresses the desired relative volume of a stream. |
| The volume-related APIs of |
| <a href="http://developer.android.com/reference/android/media/AudioManager.html">android.media.AudioManager</a> |
| operate in volume indices rather than absolute attenuation factors. |
| </dd> |
| |
| </dl> |
| |
| <h2 id="srcTerms">Sample Rate Conversion</h2> |
| |
| <p> |
| For terms related to sample rate conversion, see the separate article |
| <a href="audio_src.html">Sample Rate Conversion</a>. |
| </p> |
| |